Although I tried UC Player and SmartPlayer, and they both basicly stink.
When just transfering files from the computer to X6, they are laggy, and it looks like **** to be honest. Although they WERE able to play most formats.

The best option would be QuickTime Player. It suits a lot of phone models plus its not consuming most of memory. It does not also destroy the quality of video as long as its in the right resolution. One more thing though, if you are planning to watch movies on your phone, best thing you need to do first is convert the file to a video which is compatible with your video player.